But if you've read your history of philosophy, you might wonder this: If dualism is just common sense, why are dualistic metaphysical systems always so bizarre? Leibniz sees a universe of "monads" that move in pre-established harmony with each other but do not causally interact. Malebranche thinks nothing has any real causal power except for God, who constantly creates the universe anew at every moment. Descartes, whose "causal interactionist" dualism might initially seem a reasonable candidate for common sense, held that nonhuman animals were mere mindless machines, incapable of conscious experience. (It's hopefully not true that Descartes tossed a cat out of a window in Leiden to illustrate his belief in this.) "Common sense" philosopher Thomas Reid attributed immaterial souls to vegetables and denied that material objects had the power even to cohere into shapes without the regular intervention of immaterial souls on their behalf.

Here's my explanation of the bizarreness of dualist metaphysical systems: Commonsense opinion is not straightforwardly substance dualist. Rather, commonsense opinion about the metaphysics of mind is an incoherent mess. Thus, it's impossible to develop a detailed, coherent dualist metaphysics that respects all the inclinations of common sense.

There are at least two broad issues on which dualistic metaphysical systems have repeatedly stumbled against common sense: the causal powers of the immaterial mind and the class of beings with immaterial minds.

The causal powers issue can be posed as a dilemma: Does the immaterial soul have the causal power to affect material entities like the brain? Both yes and no answers lead to trouble. If yes, then physical entities like neurons must be regularly and systematically influenced by immaterial events. A neuron must be caused to fire not just because of the chemical, electrical, and other physical influences on it but also because of immaterial happenings in spiritual substances. Either events in the immaterial realm give it some physical or quasi-physical push that leads it to behave other than it would without that immaterial push -- which seems to violate our commonsense ideas about nonmiraculous causes of physical movement (and a minimal commonsensical deference to mainstream physics and neuroscience) -- or the immaterial causal influence somehow operates on the physical despite the fact that the physical would behave no differently absent that influence, which seems an equally strange view. Suppose, then, the other horn of the dilemma: The immaterial soul has no causal influence on physical events. If immaterial souls do anything, they engage in rational reflection. On a no-influence view, such rational reflection would have no power to causally influence the movements of the body. You can't make a rational decision that has any effect on the flow of the physical world, including the movements of your own body. This again seems bizarre by the standards of ordinary common sense.

The scope-of-mentality issue can be posed as a quadrilemma: Either (a.) among Earthly animals, only human beings have immaterial souls and they have those souls from birth (or maybe conception), or (b.) there are sharp boundaries in phylogeny and development between ensouled and unensouled creatures, or (c.) whether a being has an immaterial soul isn't a simple yes-or-no matter but rather a gradual affair, or (d.) panpsychism is true, that is, every being has, or participates in having, an immaterial soul. Each possibility violates common sense in a different way. Since on a substance dualist metaphysics of mind, the immaterial soul is the locus of mentality and conscious experience, option (a) denies dogs and apes mentality and conscious experience, contrary to what seems to be the clear opinion of most of humankind. Option (b) requires sudden saltations in phylogeny and development, which seems bizarre given the smooth gradation of differences in behavioral capacity, both developmentally and across the range of non-human animals, and given the work the immaterial soul must do if it's not to be otiose. Option (c) appears incomprehensible from a commonsense point of view: What would it mean to sort of, or kind of, or halfway have an immaterial soul? (Would you sort of go to Heaven? Even Dog Heaven, which might be a "sort of" Heaven, seems to require dichotomously either that dogs are materially instantiated there or that they have some immateriality that transcends the grave.) And despite a certain elegance in panpsychism, the idea, in option (d), that even vegetables and bacteria and proteins and thermostats have immaterial souls, or alternatively that they participate in a single grand immaterial soul, seems bizarre on the face of it, by the standards of ordinary common sense.

Any well developed metaphysical substance dualism must make choices on such matters. And all the choices seem weird. If you think otherwise, I suspect philosophy has dulled your sense of what's weird. But weird does not imply false! We have good independent reasons to think, on physical and cosmological grounds, that the world is a pretty weird place, not well matched with our commonsensical intuitions about what must be so.

The “quadrilemma” has been around for a long time. See my piece on Bayle in Justin Smith, ed. The Problem of Animal Generation in Early Modern Philosophy. Or read Bayle himself, the article “Rorarius” in his Dictionnaire historique et critique (also in translation in Historical and critical dictionary: selections from Hackett). As for the anecdote about the cat, you’ll notice that Grayling gives no source. Lots of anecdotes of this sort about Descartes and Cartesians were circulated by their enemies; the truth of any one of them, if not otherwise attested, may reasonably be doubted. Given that there is plenty of trustworthy evidence of Descartes’ cruelty to animals in vivisections, mere rumors like this are gratuitous.

Dennis: Yes, I don't claim novelty for the quadrilemma. I'll have to go back to the Bayle (and I'll check out your article too). I regularly teach a few portions of the Dictionary, which I very much enjoy, but I had either never read or forgotten about that bit!

I'd be interested to hear the provenance of the rumor about the cat, if you know it. I had certainly heard of it before reading the Grayling.

Just a random thought. I was reading some quantum physics stuff recently and apparently the 2nd law of thermodynamics just isn't true. "missing mass" has apparently been well documented at a quantum level. since mass and energy are identical in physics, this broadly suggests that some energy/mass enters an leaves the physical world in a way that we don't understand in the least. Then of course there is the fact that 70 percent of the Universe is something called dark energy which we know nothing about whatsoever. Maybe the souls just hang out in dark energy land and "doing" the physical world at a quantum level. Just sayin'...the usual causal closure stuff against (dualist) mental-physical causation just isn't as obviously true...

I buy that possibility, not necessarily involving dark energy. The question is, how and where such an agent could interact with the brain. Eccles is the only scientist that has proposed a possible mechanism, of course, not proven.

Anon: Although I am skeptical of all nonmaterialist cosmologies, I fully agree that enough doubts remain about the interpretation of quantum mechanics and other aspects of very small scale and very large scale physics and cosmology that we shouldn't take for granted the causal closure of the material/physical (neither "material" nor "physical" seems quite the right word).

Once upon a time it was the norm for people to embrace what I call "animistic dualism" - the belief that something has to be added to mere matter to account for life. There are still some ... what's the polite description? ... uneducated people who think this is true. Among the scientifically literate, animistic dualism is out of fashion. But one can still find advocates of cognitive dualism - the belief that something has to be added to mere matter to account for cognition (or consciousness, or rationality, or free will, ...).

Well, suppose one decides to be some kind of cognitive dualism. Some account is owed of the non-material component and its relation to the material. But such an account can't be constructed by empirical science. Instead, it seems to depend on thinking hard about what it is supposed to do, and then saying, "well, look, the soul (or whatever) is that which is responsible for x. What are its positive properties other than x? Well, we can't know directly, but perhaps they include y and z, or possibly even q!"

@ Eric: Not too glib! A little uncharitable, perhaps, but charitable philosophy has its downsides: People, including famous philosophers, really aren't very good at abstract thinking and metaphysical architecture.

@ Jackson: "Dualist" = "More-than-one-ist", so pluralists count, as long as the substances really are different enough.

Maybe the common sense view is better described as being realistic about the mental. Introspection and reflection are taken to be reliable guides to the nature of the, thus leaving open a variety of metaphysical options (though also excluding some (all?)varieties of materialism.

Anon: The difference between "idealism" and "dualism" is a bit tricky in the German tradition, but the same issues arise for idealists so it doesn't matter much to the current point.

I agree that common sense is realistic about the mental, and perhaps neither dualism nor materialism per se conflict with it. However, I am inclined to think that both dualist and materialist metaphysical systems face choice points where any decision is going to have consequences at conflict with common sense, as with the current quadrilemma.

In consideration of Bloom's proposal, Edward Slingerland has recently focused on the word xin (heart-mind) in pre-Qin Dynasty, arguing for a "sloppy" or "weak" form of folk dualism, rather than a Cartesian one. I think his work on this is expanding. I thought this might be of interest to you. (reported at CogSci2011 and International Association for the Cognitive Science of Religion)

Common sense tells me that all biological forms make strategic decisions based on acquired instincts reinforced by culture and/or learning. Are not all such strategies immaterial and at the same time causative?

What exactly is the "animistic dualism" allegory supposed to show, Eric? We have one case where a phenomenon was thought to require immaterial causality, so this other phenomenon must also be this way? If that's the case you mine as well argue from the case of the ancients who thought lightning was shot down by Zeus.

Not to mention the rather contentious name you've given a view everyone else knows as vitalism. I suppose it's not surprising given your guilt-by- association attacks on dualism in the beginning. It's not as if naturalistic philosophers haven't been troubled by Humean problems of causality and the determination of which things are conscious and which are not.

@shane: Animism (or vitalism) is a form of dualism because it supposes that life is a result of two things - a body and an animating principle, or soul ("anima"). The concept of the soul was very often invoked in the ancient world as providing an answer to the question of what differentiates living from non-living things. Not all accounts of the soul were thoroughly immaterialist. The animistic use of the concept of the soul has largely been eclipsed by biochemistry. But the soul has also been historically blended in with attempts to account for cognition. Cartesian dualism is, perhaps, the clearest such use.

So, if we no longer need the soul as an explanatory principle of life, it is worth asking why we should retain it as an explanatory principle of cognition (or consciousness, or perceiving qualia, or...). This is especially of concern, I think, if there is little positive account of just how the soul explains thought (other than the Cartesian answer that thinking is the soul's essence).

Yes, I have ignored more nuanced non-Cartesian forms of dualism, property dualisms, epiphenomenalist dualisms, etc. That's why there was the "Too glib?" bit in there. I was working off Eric Sw's statement that: "it's impossible to develop a detailed, coherent dualist metaphysics that respects all the inclinations of common sense."

I actually think it's quite hard to banish all vestiges of dualistic inclination from the ways we think and talk about people. But I don't think (as some people I know do) that this is evidence that favors the truth of dualism.

"Does the immaterial soul have the causal power to affect material entities like the brain?"

I would say yes. There is plenty of research in which structural changes in the brain were made with nothing more than thinking in different ways.

For example, this study (http://www.sciencemag.org/content/323/5915/800.abstract) showed that cognitive training brought on an increase in the number of dopamine receptors.

One could say that being exposed to whatever materials the subjects were training with caused the changes, not the mental activity itself. However, it seems perfectly reasonable to me that if a person chose to train their brain without any external stimuli (like multiplying triple--digit numbers in their head or trying to remember a series of numbers - tasks that increase working memory)they would reap the same benefits as if there had been external stimuli. They would have better working memory and probably more dopamine receptors. And the changes would have been prompted with nothing more than thinking, which would fall in the domain of the immaterial.

There's an even better example. A neurologist was treating his patients (I can't remember if they schizophrenia, panic disorder or OCD) by showing them their CAT scans, pointing to the neuroanatomical source of their problem, and telling them that when (let's say) a panic attack starts, they should tell themselves it not them who are sick, it's just a part of their brains which is external to them. After a few months of such thinking the patients' attacks lessened, and a new CAT scan revealed that the responsible brain part is no longer as connected to the rest of the brain. Wouldn't that also point to the influence of the immaterial "soul" on the brain? And in this case, no external stimuli could have been responsible, because the patients were thinking on their own.

The presentation is no longer on my college's website, so I can't find the reference, but if you're interested, I can ask my professor to send it to me so you can check it out.

Thanks for the comment, Borna! This kind of evidence is completely consistent with materialism, and heartily endorsed by materialists. They will say that the cognitive processes that instantiate that cognition are themselves material, so it's all a matter of material-material influence -- no immaterial soul needed!

Why do materialists see cognition as material? We have the experience of cognition, and that experience is something not reachable through exploring matter and it's properties. Experience of cognition is different from observing which neurons are fired.

While we look at a neuron firing, we don't know if there's an experience behind it or all we see is all there is. But if someone is experiencing that firing, we won't get to know about it regardless of what we observe about matter.

It's still perfectly possible that experience can only emerge when we have proper brain structures, and that this experience is gone once you remove the structures or die. But at least while we have brains, there's more to it than rock-like matter.

In other words, even if it is possible for a creature to do everything we do without having any experiences, the need for mind/soul comes from the fact that everyone can experience invisible sensations, ideas and feelings for themselves (at least I suppose they can).

I'm sure arguments such as this one have been made centuries ago, but I would still like to hear your take on it.

To the argument that one can know that one is in a mental state without knowing one is in a brain state, the standard materialist reply is that it doesn't follow that the two aren't identical. Compare Oedipus on Jocasta and "my mother". To the argument that it's possible for a creature to do everything material that we can do (e.g., bodily motions) without having consciousness, materialists generally deny the premise: If a creature did do everything material that we do it would necessarily have consciousness. There's quite a bit of material on all this. David Chalmers's anthology Philosophy of Mind is a good place to start.